1. Field of the Invention
This invention relates generally to forming spline teeth or gear teeth, and more particularly to cold extruding helical teeth in a gear blank workpiece.
2. Description of the Prior Art
Planetary gear units of the type used in automotive transmissions include ring gears having internal helical teeth rather than straight gears even though helical gear teeth are more difficult to form. The internal gear teeth must be formed with very precise dimensions and spacing in order to perform correctly.
The helical teeth may be formed by broaching, which is a cutting process in which a large broaching bar with cutting teeth is pulled through a gear blank to form the teeth. Broaching is a costly process, which requires a significant investment in dedicated machinery, lead bar, cutting tools and cutting oils. Broaching can only be applied to parts accessible in both axial directions since the long broach bar must be pulled through the inside of a gear blank to cut the teeth.
The helical teeth may be formed by gear shaping, another cutting process used to fabricate internal helical teeth. Although it is a slower process than broaching, it can be used to form blind end as well as through parts for high volume production. Even so, this process also requires an investment in expensive machinery and cutting tools.
Helical teeth may be formed by cold extrusion, in which the teeth are formed, rather than cut, into the part. A precision ground, hardened mandrel formed with external helical die teeth is forced into a workpiece, whose internal surface is formed with the negative contour of the die teeth. When helical teeth are being extruded, the mandrel must be guided in a helical path through the workpiece. This guidance combines axial translation and rotation about a central axis.
According to conventional practice, extrusion of helical ring gears requires a specific helical lead guide as part of each tool set to produce gear teeth at the proper helical angle. The lead guide is an expensive, large element of the die set and must be machined to precise dimensions. The lead mechanism requires a significant portion of the vertical dimension of the die set, and increases the total size of the hydraulic press. A lead guide and broach bar must be held in inventory for each product being made.
A need exists in the metal forming industry for an efficient, reliable technique for extruding internal and external helical gear teeth without using a lead guide to control the helical path of the mandrel through the material of the workpiece.